Audio Systems: receiver, Sony STR-DE305, sound cutting out, amplifier section, rms power

Question
Receiver is at least 5 years old, probably 10 or 12, maybe as old as 14. User's manual is copyrighted 1996. The problem began as sound cutting out after it was in use for maybe an hour or so. If I changed the source (i.e., switched from one CD carousel to another by using the selector switch on the receiver) the sound would come back on. Maybe an hour later it would cut out again. But switching back to the other CD player would instantly restore it. Note I am not talking about turning the CD players on and off. They are carousels and I left them both playing. So it isn't the players that are malfunctioning. The cycle could be repeated as long as you wanted. The interval before the sound cut out has gotten gradually shorter over maybe 3 months and now it doesn't work at all. Is this likely to be worth repairing? I'm not sure but I think this is a medium-low end receiver the equivalent today probably around 300 - 400 usd. It is "rated 100 watts per channel RMS power, with no more than 0.1% total harmonic distortion from 250 milliwats to rated output" and under Amplifier section of specs (per user's man) says dynamic power output at 8 ohms: 150 w + 150 w. If it is something simple I'm not afraid to open a case and swap a component or two. But I'm not averse to taking to a pro if they could do in a half hour what would take me a week.  

Answer
Yes, it is an old model and is not, in my opinion, what would be called a vintage or highly treasured one. In other words, it is not one that would suggest any large investment it its repair.

The first guess from an outsider would be the cause of the audio drop out being something in the switching unit such as heat causing the contacts of the switch to go open.  But, this would be wrong as the source switching is done in logic and the selector buttons are merely like a keypad which sends a logic code to an IC which generates the actual switch function.

Therefore, best guess is the IC401 called the "function" IC is either bad, going bad or has a cold solder joint that is sensitive to temperature or time or something.

Repair might be just reheating the solder joints on the bottom of IC401 or, in the worst case replacing that IC.

Of course, upon testing it may be something else entirely such as the electrolytic cap in the power supply that feeds the dc voltage to that IC dropping too low to sustain it and when you switch to another mode it recharges the cap and it goes another cycle.

Or, it could even be the clock signal coming from the main CPU failing over time which causes IC401 to drop out.

So, concluding, there are many possible causes of the symptoms you are experiencing;  I suggest the first choice of a cold solder joint around IC401 being the most probable..........

Hope this helps.

PS:  I pro may be able to test and find the problem instantly, but it is problematic because until it fails there is no way for the tech to determine any fault;  rather, the tech must wait until it fails in order to start a troubleshooting cycle.